RFR: 8262398: Shenandoah: Disable nmethod barrier and stack watermark when running with passive mode

[Roman Kennke]

On Thu, 25 Feb 2021 20:06:40 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> nmethod barrier and stack watermark allow GC not to process nmethods at GC pauses, and aim to reduce GC latency, they do not benefit STW GCs, who process nmethods at pauses anyway.
> 
> Test:
> 
> - [x] hotspot_gc_shenandoah
> - [x] tier1 with -XX:+UseShenandoahGC
> - [x] tier1 with -XX:+UseShenandoahGC -XX:ShenandoahGCMode=passive
> - [x]  tier1 with -XX:+UseShenandoahGC -XX:ShenandoahGCMode=passive -XX:-ShenandoahDegeneratedGC

If it is possible at all, it would be most useful if we could have flags like ShenandoahNMethodBarrier and ShenandoahStackWatermark, and let them drive enabling and disabling of the corresponding barriers/features. It would be even better, if we could configure concurrent modes such that they do STW class-unloading and stack-processing (if possible with reasonable amount of work). That would help ports where those features are not yet present (current ports under development with the problem are PPC and Graal).

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PR: https://git.openjdk.java.net/jdk/pull/2727